The invention is directed to a process for the conversion and utilization of silicon dioxide containing waste flue dusts from the production of silicon metal and silicon alloys to crystalline zeolitic molecular sieves of type A.
There are known various processes for the production of alkali silicates whose aqueous solutions are employed in combination with sodium aluminate and sodium hydroxide in special precipitation methods for the recovery of zeolites:
In the melt process, which is predominantly practical today, silicon dioxide (quartz) is reacted with soda (sodium carbonate) in a melt at high temperature. This melt process has the disadvantage that there is required considerable energy input to carry it out. Besides the melt process is expensive in regard to apparatus and therefore, requires considerable capital. On account of the high operating temperature, the molten compositions, furnace ceiling and recuperators are subjected to high wear and tear. Besides the melt process is characterized by an especially careful selection of the quartz sand employed, particularly in regard to the iron and aluminum content (Ullmann's Enzyklopadie der technischen Chemie, Vol. 15, 3rd edition, pages 732-733).
In the wet decomposition process, which is also known, the reaction is difficult to carry out quantitatively since in using natural sand it is only reacted relatively slowly and incompletely. According to this wet decomposition process it is only possible to operate discontinuously in an autoclave and therewith intensive as to working and energy. The discharged waterglass is dilute and in transportation leads to increased costs (Ullmann's Enzyklopadie der technischen Chemie, Vol. 15, 3rd edition, page 736).
To partially avoid the above-mentioned disadvantages there is used as the starting material for the production of alkali silicates in Japan naturally occurring volcanic sand.
It has also been proposed already to use for the production especially pure alkali silicates, particularly for industrial purposes, pyrogenically produced, chemically high pure, amorphous silica as the starting product (Ullmann's Enzyklopadie der technischen Chemie, Vol. 15, 3rd edition, page 735). This method of production, however, has the disadvantage that because of the high cost of starting material the recovered alkali silicate solution can be employed for special purposes, however, not on a technologically broad basis.
To recover waterglass solutions from industrial waste products there has already been proposed the working up of waste silica as it precipitates in the production of Na.sub.2 AlF.sub.6, AlF.sub.3, NaF and HF from the hexafluorosilicic acid, H.sub.2 SiF.sub.6, arising out of the raw phosphate process (Biegler, German Pat. No. 2 219 503). However, there is obtained only a waterglass with limited stability since the incompletely removed fluoride ion catalyzes the polymerization of the waterglass. Besides the content of toxic fluoride is disturbing in the subsequent working up of this material.
Recently there have been attempts to work up other high silica containing waste products to form waterglass. Thus, in the recovery of silicon carbide which is produced in the electric furnace by metallurgical process there is formed as the reaction product a large amount of wet gases which initially contain silicon monoxide which then is oxidized with the entrance of air oxygen to highly dispersed silicon dioxide. The reaction gases additionally also contain solid impurities which together with the silicon dioxide is separated in considerable amounts as flue dust from the waste gases. The resulting flue dust possesses a high content of amorphous SiO.sub.2. As impurities besides other oxides such as Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgO and CaO there is also found carbon used in the form of graphite in the electrodes. It enters in finely divided form in the waste gas and gives the flue dust a black-gray color. Furthermore, the eliminated products contain impurities originating from the reaction of quartz with, for example, oil coke. Furthermore, there are also contained in the flue dust organic materials formed by thermal decomposition out of the coal tar or dextrin used as binders in the electrode material, which arrive in the SiO.sub.2 containing waste gas as cracked products. These materials are firmly adsorbed by the extremely finely divided silica in the waste dust.
Waste flue dust also accumulates in considerable amounts in the production of silicon-iron alloys. Thus, there is formed in the reducing production of one ton of silicon-iron alloy 0.2-0.5 ton of dust. The reuse of this dust by recycling in the electric furnace to be sure has been tried but because of the necessity of a granulating process has little profitability. Since until now there has been no significant industrially or economically practical possibility of use the waste flue dust according to the state of the factory is either given off to the atmosphere as a white smoke (aerosol) and forms the basis of a severe air pollution or it is collected in the sea and rivers or is brought to deposits.
To be sure there has already been proposed the chemical preparation of silica containing waste flue dusts by decomposition with alkali hydroxide in the wet process. Thereby there is produced from the unpurified waterglass solutions obtained silica gels with the aid of acids. These silica gels, however, have only limited use because of the previously mentioned solid and soluble impurities, Japanese opened Sho No. 49-134599 and Japanese opened Sho No. 49-134593.
In the earlier unopened German patent application No. P 26 09 831.7 there is described a process for the preparation of precipitated silicas and silicates from silicon dioxide containing waste flue dusts employing the following combination of process steps:
I. Dissolution of the flue dust in alkali hydroxide solution with formation of an alkali silicate solution with high module (SiO.sub.2 :Na.sub.2 O);
II. Purification of this alkali silicate solution from organic constituents by treatment with activated carbon and/or an oxidizing agent and separation of the non-decomposable black residue from the solution; and,
III. Reaction of the purified alkali silicate solution with acids and/or salts of aluminum and calcium or magnesium at temperatures in the range of 60.degree.-110.degree. C. in the pH range of 1-12, subsequent filtration, washing and drying of the filter cake paste as well as comminution of the drier granulate to recover finely divided, amorphous, pure precipitated silica.
Although with the help of this combination of steps for the first time at present there is the possibility to convert the environment loading flue dust to an industrial use, further endeavors are necessary to increase meaningfully by chemical change the dusts whose toxicological hazards have recently been published (J. C. A. Davies, The Central African Journal of Medicine, Vol. 20 (No. 7), July 1974, pages 140-143 and D. M. Taylor, J. C. A. Davies, The Central African Journal of Medicine, Vol. 21 (No. 4), April 1975, pages 67-71).
The invention therefore is based on the problem of developing a chemical process for the preparation of crystalline zeolitic molecular sieves of type A from silicon dioxide containing waste flue dusts from the production of silicon metal and silicon alloys. Type A zeolite molecular sieve is a very valuable product with extensive possibilities for use, e.g., as an ion exchanging heterogeneous inorganic builder in modern washing agents, as adsorption agent for the drying of gases and liquids, as separating agent for the decomposition of gas mixtures, as catalysts and as catalyst constituents, as ion exchanger for drinking water and industrial waste water.